The present invention relates to brazing materials for components that operate at high temperatures. More particularly, this invention relates to nickel-base braze alloys that exhibit sufficient strength and oxidation and creep resistance for use as a filler material for holes in a turbine blade, such as holes in high pressure turbine blade tip caps.
Components of gas turbine engines, such as blades (buckets), vanes (nozzles) and combustors, are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical properties for turbine operating temperatures and conditions. Because the efficiency of a gas turbine engine is dependent on its operating temperatures, there is a demand for components, and particularly turbine blades and vanes, that are capable of withstanding increasingly higher temperatures. As the maximum local metal temperature of a superalloy component approaches the melting temperature of the superalloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine engine blades and vanes often require a complex cooling scheme in which bleed air from the engine compressor is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface. Considerable cooling air is often required to sufficiently lower the surface temperature of a blade or vane.
Blades and vanes formed by casting processes require cores to define the internal cooling passages. During the manufacture of high pressure turbine blades of the type as disclosed in commonly-assigned U.S. Pat. No. 5,232,343, holes in the blade tip cap are required to locate the cores. Before engine installation, these holes, which may have diameters of, for example, about 0.030 to 0.040 inch (about 0.76 to 1.02 mm), must be securely closed to prevent the loss of cooling air through the tip cap. As represented in FIG. 1 and reported in commonly-assigned U.S. Pat. No. 6,187,450 to Budinger et al., an existing technique is to fill a tip cap hole 10 by injecting a slurry 12 containing a mixture of particles 14 and 16 of two different alloys, one of which (e.g., 16) has a lower melting point. Budinger et al. reports the higher melting alloy as constituting about 45 weight percent of the particulate mixture, with the balance being the lower melting alloy. Budinger et al. also describe the higher melting alloy as containing (by weight) about 0.15-0.19% carbon, about 13.7-14.3% chromium, about 9.0-10.0% cobalt, about 4.8-5.2% titanium, about 2.8-3.2% aluminum, about 3.7-4.3% tungsten, about 3.7-4.3% molybdenum (7.7% minimum tungsten+molybdenum), the balance nickel and incidental impurities, which is similar to the superalloy known as René 80. (As used herein, incidental impurities are those elements that may be difficult to completely eliminate from an alloy due to processing limitations, yet are not present in sufficient quantities to significantly alter or degrade the desired properties of the alloy.) The lower melting alloy is reported as containing (by weight) 0.05% maximum carbon, about 14.8-15.8% chromium, about 9.5-11.0% cobalt, about 3.0-3.8% tantalum, about 3.2-3.7% aluminum, about 2.1-2.5% boron, the balance nickel and incidental impurities. During brazing, only the lower melting particles 16 are melted, forming a liquid that fills voids between the higher melting particles 14 and, on solidification, bonds the high melting particles 14 together within the tip cap hole 10 and to the substrate material 18 surrounding the hole 10. The resulting brazement 20 is represented in FIG. 2.
Budinger et al. teach that filling a tip cap hole by injection of a slurry containing a mixture of higher and lower melting particles can lead to incomplete filling and a high internal porosity level (as depicted in FIG. 2) that increases susceptibility to oxidation. More generally, there tends to be a tradeoff between oxidation behavior and mechanical properties, in that braze compositions suitable for filling tip cap holes often achieve improved mechanical properties at the expense of oxidation resistance, and vice versa. Braze compositions with low oxidation resistance oxidize away during service, while those with insufficient mechanical properties tend to be ejected from the hole. In both instances, the effect is to re-open the holes during service.
As a solution, Budinger et al. teach filling a tip cap hole 10 with a first slurry 22 containing particles 24 of a relatively high melting point alloy, and then covering the hole 10 and the first slurry 22 therein with a second slurry 23 containing particles 26 of a lower melting point alloy, as represented in FIG. 3. When heated to a temperature above the melting temperature of the lower melting particles 26 but below the melting temperature of the higher melting particles 24, the molten lower melting alloy completely infiltrates the higher melting particles 24 within the hole 10 so that, on solidification, the filling formed by the lower melting particles 26 bonds the high melting particles 24 within the tip cap hole 10, yielding the brazement 30 depicted in FIG. 4. Budinger et al. disclose a suitable higher melting alloy as containing (by weight) about 11.45-12.05% cobalt, 6.6-7.0% chromium, 5.94-6.3% aluminum, 1.3-1.7% molybdenum, 4.7-5.0% tungsten, 6.2-6.5% tantalum, 2.6-3.0% rhenium, 1.3-1.7% hafnium, 0.10-0.14% carbon, up to 0.02% titanium, the balance nickel and incidental impurities (similar to the superalloy known as René 142, U.S. Pat. No. 5,173,255). Budinger et al. disclose two suitable lower melting alloys that exhibit improved capillary flow for yielding a more fully dense structure. A first of the lower melting alloys contains (by weight) 0.13-0.19% carbon, about 13.7-14.3% chromium, about 9.0-10.0% cobalt, about 4.6-5.2% titanium, about 2.8-3.2% aluminum, about 0.5-0.8% boron, about 4.2-4.8% silicon, and the balance nickel and incidental impurities (essentially René 80 modified to contain silicon and have a higher boron content). The second lower melting alloy contains (by weight) 0.01% maximum carbon, about 18.5-19.5% chromium, about 0.03% maximum boron, about 9.8-10.3% silicon, and the balance nickel and incidental impurities. Finally, Budinger et al. disclose that up to fifty weight percent of the lower melting particles 26 in the second slurry 23 can be replaced with particles 24 of the higher melting alloy.
The approach taken by Budinger et al. was to reduce the reliance of boron as the melting point depressant through additions of silicon. It was concluded that, though imparting strength, boron has a negative effect on oxidation resistance. While successfully addressing the concern for incomplete filling of a tip cap hole, the compositions taught by Budinger et al. have been found to be susceptible to oxidation, and in some cases exhibit insufficient mechanical properties, particularly creep rupture life. Therefore, there remains a need for a tip cap hole braze material capable of exhibiting further improvements in environmental resistance and mechanical properties, as well producibility in manufacturing.